Liver disease is a cause of substantial morbidity and mortality in the U.S. and cholestasis (impairment of bile flow) is a common and devastating manifestation of liver disease. This is an application to renew a grant focused on increasing our understanding of the biological causes of cholestasis, and more specifically, the role of the FIC1 (ATP8B1) protein in health and disease. Mutations in the gene encoding FIC1 (a P-type ATPase) have previously been identified as the genetic etiology of some cases of progressive familial intrahepatic cholestasis (PFIC) and benign recurrent intrahepatic cholestasis (BRIC). A mouse model of FIC1 disease has been generated and partially characterized, resulting in the novel finding that excessive bile salt resorption may contribute to development of cholestasis. The phenotype of Fic 1-deficient mice differs depending upon the murine strain background, indicating the existence of genetic modifiers of the murine Fic 1-deficient phenotype. Similarly, human patients who share the same FIC1 mutation can have widely varying severity of disease. Mapping and identification of modifier loci for the murine Fic 1-deficient phenotype will illuminate the biological role of FIC1, and may identify candidate susceptibility loci for development and progression of FIC1 disease, as well as more common disorders of complex etiology. This proposal focuses on use of genetic and candidate gene approaches to identification of modifier loci influencing strain-specific phenotypic differences, including weight at weaning, and response to bile salt supplemented diet, in Fic1 mutant mice. In Specific Aim 1, we will perform further phenotypic characterization of the Fic1 mutant mice in C57B1/6, 129S1, and 129S4 strain backgrounds. Results of these studies will inform Specific Aims 2-4. In Specific Aim 2, we will genetically map modifier loci for phenotypic traits of Fic1 mutant mice that differ between the C57B1/6 and 129 strains. In Specific Aim 3, we will use genetic, candidate gene, and other approaches to further characterize 3 of these loci, with the goal of identifying a strong candidate gene for one or more of the modifier loci. In Specific Aim 4, we will more precisely define the genomic regions thought to differ between the 129S1 and 129S4 strains, and perform targeted studies to correlate phenotypic and genotypic findings in Fic1 mutant mice of 129S1, 129S4, and mixed 129S1;129S4 strain backgrounds.